Photonic integrated circuits are typically comprised of a plurality of photonic devices, located on a semiconductor substrate, that are in optical communication with one another. Most methods for creating photonic integrated circuits involve forming one photonic device at a time. This is due to an inability to regionally vary the bandgap of the quantum well (QW) material being deposited in a given epitaxial growth.
In the methods noted above, the epitaxial layers required to form a first type of photonic device, such as a laser, are grown over the whole substrate. The growth times and source material concentrations used for the growth are selected so that the quantum well (QW) material that is deposited has the requisite characteristics, i.e., band gap, to function as the desired device. The layers are then masked at the region where the first photonic device is desired. Subsequently, the layers in unprotected regions are etched away where other devices, such as modulators or waveguides, are desired. After etching, layers corresponding to a second type of photonic device are grown on the substrate in the etched regions. Growth conditions are adjusted for the second growth so that the QW material exhibits the appropriate band gap. If a third type of photonic device is desired, the layers are again masked and etched, conditions are adjusted and a third series of epitaxial layers are grown in the etched region.
Methods that utilize successive growths as described above may be collectively referred to as "etch and regrow" methods. Etch and regrow methods prevent devices such as lasers or other active elements from being fabricated at the same time and in the same optical plane as other devices such as waveguides and optical gratings because such devices require QWs with different bandgaps. Moreover, devices grown from the etch and regrow method frequently exhibit poor optical interface quality between different devices, which can result in internal reflections and coupling losses.
Selective area epitaxy (SAE) is an epitaxial growth method that minimizes the poor optical interface problems associated with the etch and regrow method. Using SAE, the bandgap of QW material can be varied in the same plane with a single growth. Thus, layers defining various photonic devices can be grown simultaneously. See Joyner et al., "Extremely Large Band Gap Shifts for MQW Structures by Selective Epitaxy on SiO.sub.2 Masked Substrates," IEEE Phot. Tech. Lett., Vol. 4, No. 9 (Sept. 1992) at 1006-09 and Caneau et al., "Selective Organometallic Vapor Phase Epitaxy of Ga and In Compounds: A Comparison of TMIn and TEGa versus TMIn and TMGa," J. Crystal Growth, Vol. 132 (1993) at 364-70, which are both hereby incorporated by reference.
In the SAE method, dielectric masks, such as SiO.sub.x or SiN.sub.x, are deposited on a substrate. Such masks typically comprise two strips spaced to form a gap. Source material for forming the epitaxial layers, such as indium (In), gallium (Ga), arsenic (As), and phosphorus (P), is typically delivered via a technique such as metalorganic vapor phase epitaxy (MOVPE).
Source material arriving from the vapor phase will grow epitaxially in regions where the mask is open, i.e, the substrate is uncovered. Source material landing on the mask itself will not readily nucleate. Given the proper temperature and mask width, most of the source material that lands on the mask reenters the vapor phase and diffuses, due to the local concentration gradient, to find an unmasked region.
Compared to a completely unmasked substrate, the QW growth that occurs in the gap for both InGaAs and InGaAsP epilayers will be thicker, and richer in indium. This effect is due to the relative diffusion coefficients of In and Ga under typical MOVPE growth conditions. As the QW layers thicken, changes occur in the quantum confined Stark effect resulting in longer wavelength (lower energy band gap) QW material. Increasing indium content also results in longer wavelength QW material. Thus, from both the quantum-size effect and change in alloy composition, the QWs in the gap are shifted to lower energy band gaps than regions far from the mask. Accordingly, the refractive index of the QWs in the gap are increased relative to the regions outside the gap. By varying the ratio of the mask width to the gap width, the composition, and hence the bandgap and refractive index, of QW material can be varied. In U.S. Pat. No. 5,418,183, for example, a SAE method is employed to fabricate lasers and passive waveguides so that their respective QWs are located in the same plane of material.
However, when optical grating, waveguides and other active devices are fabricated together by etch and regrow techniques, they are either formed in different planes or with significant loss due to interfacial reflections in the plane of propagation.